An AC/DC power converter is typically used to convert a commercial AC power source into a DC power supply of a specific voltage. Referring to FIG. 1, at the AC power input terminals 12 and 14 of an AC/DC power converter for connecting to an AC power source 10, there is an AC/DC power interface 16 which includes a bridge rectifier 18 to rectify the AC voltage VAC supplied by the AC power source 10 into a DC input voltage Vin for the AC/DC power converter to further converter into a DC output voltage. The AC/DC power interface 16 also includes an EMI filter capacitor X-CAP connected between the AC power input terminals 12 and 14. However, the presence of the EMI filter capacitor X-CAP brings a risk of electric shock because once the AC power source 10 is removed, the EMI filter capacitor X-CAP will sustain the voltage of the AC power source 10 that occurs at the instant moment when the AC power source 10 is removed, and may be up to hundreds of volts. For this issue, conventional solutions to comply with safety specification NE60950 or IEC950 are to connect a bleeding resistor Rb parallel to the EMI filter capacitor X-CAP, such that upon removal of the AC power source 10, the bleeding resistor Rb and the EMI filter capacitor X-CAP establish a circuit loop for discharging the EMI filter capacitor X-CAP. Nevertheless, the presence of the bleeding resistor Rb connected between the AC power input terminals 12 and 14 establishes a normally conductive current path between the AC power input terminals 12 and 14, resulting in power loss Ploss=VAC2/Rb as long as the AC power source 10 is connected with the AC power input terminals 12 and 14, and thereby reducing the efficiency of the AC/DC power converter. In addition, IEC950 requires the discharge time of the EMI filter capacitor X-CAP be shorter than the time constant of one second. Therefore, to conform to this safety specification, the larger the EMI filter capacitor X-CAP is, the smaller the bleeding resistor Rb must be, and yet the smaller this resistance, the greater the power loss caused by the bleeding resistor Rb. For example, under VAC=230V, if X-CAP=5 μF and Rb=150KΩ, then Ploss=353 mW. In other words, power loss of 353 mW occurs even when the AC/DC power converter is at no loading state or in a standby mode. While specifications on power loss were relatively loose in the past, the rising awareness of environmental protection has led to more tight requirements on power loss; however, power loss resulting from the bleeding resistor Rb has hindered compliance of the AC/DC power converter with today's much stricter environmental protection specifications.
U.S. Pat. No. 7,046,529 uses the circuit of the AC/DC power converter to generate a control signal for switching a plurality of resistors in the AC/DC power interface 16 between a plurality of configurations. Once the AC power source 10 is removed, the resistors are reconfigured to establish a smaller equivalent resistance as the bleeding resistor Rb to discharge the EMI filter capacitor X-CAP within a required time period. When the AC power source 10 is connected, the resistors are reconfigured to establish a larger equivalent resistance as the bleeding resistor Rb to reduce power loss. However, this approach does not eliminate the use of the bleeding resistor Rb, and thus power loss still occurs. Moreover, this approach requires modification of the circuit of the AC/DC power converter and is thus not applicable to existing AC/DC power converters unless the circuits of existing AC/DC power converters are redesigned.